Presently, the combustion at low temperature is receiving a great deal of interest because emissions of micro- and nano-pollutants are expected to be greatly reduced. Following previous studies on the low temperature combustion behavior, the authors report results and discussion of steady-state experiments on an atmospheric, pre-pilot scale, 140 mm ID, FB reactor, equipped with an under-bed, air-assisted, liquid-fuel injector. The experimental program was focused on the operation at temperatures lower than the classical value for FBC of solid fuels (i.e., 850°C). The data series taken into consideration are the concentrations of the main unburned species in the splash zone, those of oxygen measured in the bed and in the splash zone as well as the freeboard pressure. The interpretation of the results is mainly based on the statistical analysis in the time domain. The combustion pattern of bio-diesel is compared to that of the diesel fuel under varying operating conditions (e.g., bed temperature, dispersion air velocity at the fuel nozzle, injector height in the bed). Conclusions that were previously published on the base of lab-scale results are checked against new data obtained on the pilot scale. An innovative technique for the analysis of the micro-explosive regime is presented. It consists in the comparison of oxygen concentration measured by the zirconia-based probes at different heights in the bed and in the splash region, pressure signals measured in the freeboard and purposely filtered, and video-recordings of the bed surface phenomena.